[Rh(nbd)2]BF4. NMR and X-ray analyses established the
molecular structure of 7aÀb (see Figure 2 for 7a and the
Supporting Information for 7b), in which the PÀOP
ligands work as bidentate ligands, exhibiting small natural
bite-angles of 82.25(4)° (7a) and 80.22(6)° (7b). Further-
more, the absolute configuration of the stereogenic carbon
at the ligand backbone was unambiguously assigned to be
(R). Regarding the coordination of 6 to hydroformylation
rhodium precursors, [Rh(acac)(PÀOP)] complexes were
also efficiently formed by the reaction of stoichiometric
amountsof ligands6 and [Rh(acac)(CO)2]. TheNMR data
are consistent with there being a bidentate coordination
between the ligating groups and the metal.11
Figure 1. Reported small bite-angle PÀOP ligands.
First, the reduction of the phosphamide 49 was evaluated
using methyloxazaborolidine (Me-CBS) as a chiral catalyst
and BH3 DMS as a reducing agent. After some experimen-
tation, the following procedure was found to provide good
results: slow addition (1 h) of BH3 DMS to a solution of the
3
3
(R)-Me-CBS catalyst (30 mol %) and 4 in THF at rt, which
afforded the phosphine-alcohol 5 as the borane adduct in
87% yield and high enantiomeric ratio (er = 84:16 in favor of
the R enantiomer,10 Scheme 1). Optical enrichment of 5 by
semipreparative HPLC on a chiral stationary phase11 af-
forded the enantiomerically pure compound (R)-5 in 77%
yield in gram amounts.12 This product was subsequently
O-phosphorylated with several bisnaphthol phosphorochlor-
idites under basic conditions. The resulting borane-protected
1,1-PÀOP derivatives were subsequently deprotected with
DABCO to afford the target ligands 6aÀe (Scheme 1).
Interestingly, ligands 6aÀe were configurationally stable:
the stereogenic carbon did not undergo any detectable
epimerization at rt (as determined by 31P{1H} NMR).
The ability of ligands 6 to coordinate to cationic rho-
dium precursors suitable for catalysis was assessed by
NMR spectroscopy and X-ray analysis. Rhodium com-
plexes [Rh(nbd)(6a)]BF4 (7a) and [Rh(nbd)(6b)]BF4 (7b)
were synthesized in almost quantitative yield by mixing
stoichiometric amounts of either ligand (6a or 6b) with
Figure 2. Crystal structure of 7a (ORTEP drawings showing
thermal ellipsoids at 30% probability). H-atoms and the BF4
counterion are omitted for clarity.
Having developed efficient syntheses for the PÀOP
ligands 6 and studied their coordination behavior with
suitable rhodium precursors for catalytic studies, the
authors then assessed their catalytic performance in asym-
metric hydroformylations and hydrogenations.
The ligands 6aÀe were initially screened in asymmetric
hydroformylation of vinyl acetate (8a), styrene (8b), and
(allyloxy)trimethylsilane (8c) to the corresponding alde-
hydes 9aÀc and 10aÀc. This chemistry was done using
preformed (in situ) [Rh(acac)(6aÀe)] complexes under
CO/H2 (1:1, 10 bar) in toluene at 40 °C (Table 1). The
results clearly indicated that catalytic performance de-
pended on both the substrate and ligand. For instance,
for substrates 8a and 8c, the rhodium complexes of ligands
6cÀe performed better than those derived from 6aÀb (in
Table 1 compare entries 1À2 with entries 3À5); however,
for substrate 8b, full conversion was achieved with each of
the PÀOP ligands. These results demonstrated that greater
steric bulkiness at the ortho-positions of the phosphite
moiety leads to higher conversions and regioselectivities
in the branched aldehydes. It should be also recalled here
that related catalytic systems derived from 1,1-PÀOP
ligands 1 (with (S)-H0- and R = H) failed in the hydro-
formylation of 8b due to catalyst decomposition.5a As
such, the presence of the CH-methyl group in the ligands
reported here increases the stability of the resulting cata-
lytic species (in Table 1 see entries 1À2 for substrate 8b).
Regarding enantioselectivity, the rhodium complexes of
ligands 6aÀc provided poor to moderate results (entries
1À3 in Table 1). However, the introduction of a more
sterically hindered phosphite group in ligands 6dÀe
ꢀ
ꢀ
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(7) (a) Fernandez-Perez, H.; Pericas, M. A.; Vidal-Ferran, A. Adv.
Synth. Catal. 2008, 350, 1984. (b) Donald, S. M. A.; Vidal-Ferran, A.;
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Rico, J. L.; Vidal-Ferran, A. Chim. Oggi 2010, 28, XXVI. (e) Nunez-Rico,
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J. L.; Fernandez-Perez, H.; Benet-Buchholz, J.; Vidal-Ferran, A. Organo-
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metallics 2010, 29, 6627. (f) Panossian, A.; Fernandez-Perez, H.; Popa, D.;
Vidal-Ferran, A. Tetrahedron: Asymmetry 2010, 21, 2281. (g) Etayo, P.;
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Nunez-Rico, J. L.; Fernandez-Perez, H.; Vidal-Ferran, A. Chem.;Eur. J.
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2011, 17, 13978. (h) Etayo, P.; Nunez-Rico, J. L.; Vidal-Ferran, A. Organo-
metallics 2011, 30, 6718. (i) Nunez-Rico, J. L.; Etayo, P.; Fernandez-Perez,
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H.; Vidal-Ferran, A. Adv. Synth. Catal. 2012,354, 3025. (j) Nunez-Rico,J.L.;
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(8) For a general review, see: Franke, R.; Selent, D.; Boerner, A.
Chem. Rev. 2012, 112, 5675 and references cited therein.
(9) Angharad, B. R.; Clarke, M. L.; Guy, O. A.; Ratcliffe, D. A.
J. Organomet. Chem. 2003, 667, 112.
(10) The absolute configuration of 5 was unequivocally established
by X-ray analysis of the rhodium complex of a 1,1-PÀOP ligand, as
shown later in the discussion.
(11) See Supporting Information for details.
(12) Although 5 was configurationally stable at À20 °C, it had
racemized to the extent of 36% after 2 months at rt.
B
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